Sixteen-year-old Bill O'Connell is a new recruit in the Royal Canadian Navy, assigned to a ship that hunts for Germany's feared U-boats. With the European mainland under Nazi occupation, safe ocean passage is critical — but the Germans are building U-boats faster than the Allies can sink them, and Britain is starved of supplies. Every gallon of aviation fuel, every explosive shell, and every can of peas sent to the British Isles from North America has to be shipped by sea, so Bill and the rest of the Allied forces have the fate of the free world resting on their shoulders. If the Allies cannot keep their merchant ships from the attacks of Germany's U-boats, the odds of winning WWII will tip in favour of Hitler.